The present invention relates generally to piston and cylinder devices, and more particularly to a lock for holding a piston in a cylinder at one of two extreme positions.
One prior device for locking a piston and piston rod at an extreme position in a cylinder is disclosed in Powers et al. U.S. Pat. Nos. 2,946,313 and 2,959,155. A latching stem is secured to a piston of a fastener driving tool. The latching stem includes an annular channel which is engaged by ball bearings mounted in the cylinder to lock the piston in place therein. The piston is unlocked by admitting pressurized fluid into the cylinder which causes a release sleeve to move upwardly. This movement allows the ball bearings to move out of engagement with the latching stem channel and permit the latching stem and piston to move within the cylinder.
A different type of locking device is disclosed in Skelton et al. U.S. Pat. No. 3,397,620. This patent discloses locking means in the form of a series of ball bearings disposed in a head or cap of the cylinder which engage a concentric groove on the end of a piston rod. The balls are urged inwardly into engagement with the groove by a cam surface of a circumferential spring-loaded ring member, also disposed in the head or cap. The locking action is overcome by the admission of pressurized fluid which acts against the ring member to move it against the spring force and release the ball bearings from the groove.
The devices shown in the above patents are quite complex in that they require a plurality of springs, complicated and/or numerous seals and/or special apparatus for mounting the various components of the locking devices to the head and/or cylinder.
Furthermore in the Powers et al. apparatus, the locking components are mounted by means of a bolt which extends through an end cap of the device, thereby providing a path for contaminants to enter the cylinder.
The apparatus shown in Skelton et al. utilizes an end plate bolted to a head or cap of the cylinder which carries the locking components. This device uses seals associated with the ring member to minimize leakage of pressurized fluid. However, these seals do not prevent contaminants from entering a spring chamber which houses springs in the end plate for loading the ring member. Also, once the ring member is moved to compress the springs, the seals create a vacuum in the spring chamber tending to oppose the force of the springs, in turn impairing the function of the device.